The tube nest consists of straight or U-bent tubes through which water or other cooling liquid is passed thus condensing the steam flowing in from the periphery on the surfaces of the tubes. Water flows in one or more directions through the tubes. The water is at all times in liquid phase and takes up the vaporization heat of the condensed steam during its passage through the tubes with consequent continuous increase in, the temperature of the water. The steam is condensed on the outer surface of the tubes while the temperature remains substantially constant. This means that the condensing capacity, which is proportionate to the difference in temperature between the steam and the water, varies along the length of the tubes, with the greatest capacity at the water inlet end and continuously decreasing capacity towards the warmer outlet end.
The driving force for the supply of steam is the pressure differential between the outer periphery of the tube nest and the ventilated air-cooling section. Since this pressure differential is substantially constant for the entire tube nest, an excess of steam will be supplied to the warmer sections of the tube nest, and a corresponding deficient amount of steam will be supplied to the colder sections. This leads to a tendency for the steam to flow from the warmer section of the tube nest to the colder section.
However, since such tendency is completely or partly prevented in the conventional constructions of heat exchangers, due to the fact, that among others, that the tubes are braced at selected locations in longitudinal direction, usually by plates, there will be a shortage of steam in the colder parts and the steam will flow into these sections through the back entrance through the air cooler portion or through overflow openings. This leads to the creation of stagnation zones in the colder sections of the tube nest, and non-condensable gases, such as air, present in the steam will accumulate in these zones and thus prevent condensation.
The aforementioned bracing of the tubes is a necessary precaution in order to prevent vibrations in the tubes and to impart stability to the tube nest. This is normally accomplished by means of transverse brace plates which are provided with bores arranged in accordance with the tube pattern and which fit about the tubes which extend through the brace plates. Such brace plates are spaced from each other in a manner suitable for supporting the tubes, but the plates will then form walls preventing the desired longitudinal distribution of the steam, which may also be prevented by other construction details in the condenser.